1. Field of the Invention
The invention relates to an apparatus for ultrasound imaging and a method therefor. Particularly, the invention relates to an apparatus for ultrasound harmonic imaging that uses linearly or non-linearly frequency modulated transmitting waveforms and a method thereof.
2. Description of Related Art
An ultrasound image is generally formed by a linearly reflected-back fundamental signal, and the fundamental signal is easy to be influenced by phase aberration to cause poor image quality. A harmonic signal is slowly generated by a sound wave transmitted in human tissues due to a finite amplitude distortion phenomenon, or is generated by intense non-linear media such as ultrasound microbubble contrast agents, etc. During tissue harmonic imaging, since an intensity of the harmonic signal is lower than an intensity of the fundamental signal in the beginning, when the sound wave is reflected back to a probe, a phase error thereof caused by a body superficial fat layer is relatively small, so that the tissue harmonic imaging is less influenced by the phase error, and has a higher image contrast resolution. Therefore, the tissue harmonic imaging is widely used in clinical diagnosis. During contrast agent harmonic imaging, the used contrast agent is composed of many microbubbles, and the microbubbles may produce resonance when being excited by the sound wave, and during the resonance, a plurality of intensive harmonic signals transmitted back to the probe is generated. Therefore, in clinical practice, the contrast agent is injected into blood vessels, so that the blood area also produces the intensive harmonic signal due to full of the microbubbles, which clarifies a vascular structure and blood perfusion, i.e. achieves a higher contrast.
A main difference between the ultrasound fundamental signal and the harmonic signal lies in a frequency range of the echo signals of the ultrasound fundamental signal and the harmonic signal. Imaging performed according to an echo signal of a frequency f0 reflected by a reflective body is referred to as fundamental imaging. The harmonic imaging is to retrieve high frequency harmonic signals with frequencies of 2f0 and 3f0 for imaging. Since the harmonic signals are obtained through non-linear response of the medium on the incident sound wave, the harmonic imaging is also referred to as non-linear imaging. Therefore, by setting a receiving range to design a low frequency or high frequency filter, it is determined whether a fundamental image or a harmonic image is finally obtained. In the harmonic signals, a second harmonic signal generally has a larger strength, which is usually used and discussed.
Although the harmonic imaging draws attention in clinical diagnosis due to its higher image quality, a weak signal strength thereof results in a fact that an image sensitivity and penetration are all subjected to a considerable influence. For example, a strongest focus of the harmonic signal is still less than the fundamental signal for more than 20 dB. Some related researches and patents focus on increasing the harmonic strength in order to obtain higher quality ultrasound images.